Cows account for 40% of all livestock emissions in Britain. Photograph: Daniel Berehulak/Getty

With humans it's just plain bad manners but with cows it's bad for the planet.

Now 200 dairy cows have been fitted with microphones to measure how much methane they belch and fart while chewing the cud.

Dairy cows account for 40% of all UK livestock emissions and 75% of the carbon footprint of milk production.

The herd of Holsteins, at Tesco's Dairy Centre of Excellence on Merseyside, has been fitted with "burp collars" to see if different feeds can cut emissions.

Motion sensors embedded in the rumination collars, as they are officially known, pick up stomach sounds, providing hourly data on the cow's digestion.

The information is transferred to the farm's computers when cows pass over an ID unit en route to the milking parlour.

Tesco says consumers want to know food's carbon footprint in addition to their nutritional content so it has produced "carbon footprint" labels for more than 130 products, with full fat, semi-skimmed and skimmed milk added last month.

The supermarket chain aims to disclose the carbon footprints of 500 products by the end of the year.